This invention relates to data signal isolation apparatus, for use particularly, but not exclusively, to electrically isolate Ethernet signals for use in an associated intrinsically safe part of a circuit to which the apparatus is applied.
Known Ethernet transceivers comprise non-intrinsically safe galvanic isolators or transformers, so when Ethernet is used in intrinsically safe environments, it is usually made safe by the use of classic Zener barriers, with the shunt connection taken to a safety Earth.
FIG. 1 shows just such an arrangement, in which a non intrinsically safe Ethernet connection 12, is isolated by a non-intrinsically safe transformer 7, and is rendered intrinsically safe by being power restricted by a classic shunt barrier comprising fuse protection 4, voltage clamping 3, and current limiting resistors 5. As such, an output connection 6 is intrinsically safe, and any over-voltage is shunted to a safety ground or a safety earth E.
However, whilst this arrangement is satisfactory for low speed data transmission, high speed performance is affected by the parasitic influence of the protection components, for example capacitance and resistance or impedance of the Zener diodes.
Furthermore, the known isolating transformer 7 does not meet the segregation requirements of the EN50020 standard, because of insufficient space for “gapping” within the former. As such, it cannot be used for intrinsically safe isolation, so an additional segregation component, which does meet the required standard, has to be added. However, this leads to an increase in the stray inductance, further decreasing signal fidelity.